The following is related generally to the optical components used in optical communication networks, and, more specifically, to optical devices that can attenuate or switch optical signals, while also providing the functions of optical power monitoring and detection.
Variable Optical Attenuators (VOAs) and optical switches are widely deployed in optical networks, typically in the 1550 nm or 1310 nm wavelength windows, as well as other wavelength ranges. In wavelength-division-multiplexed optical networks where multiple wavelengths are used, so that multiple channels of information can be transmitted or carried on a single fiber, Variable Optical Attenuators are used at various points in the network, to manage the optical power of the multiple optical signals or wavelengths. Optical signals that are entering the network at a network node may need to have their optical power adjusted to roughly match the optical power of signals that are already present on the fiber. Optical amplifiers may be deployed at various points in the network, to increase the optical power of signals that have traveled down their respective optical fibers for long distances. In general, optical amplifiers increase the power of all wavelengths that are present on a fiber, but the amount of amplification or gain may not be well-controlled, and may not be the same for all wavelengths. For this reason, and others, Variable Optical Attenuators may be used to adjust the optical power level of signals that have been amplified by optical amplifiers. Individual wavelengths that are dropped at a network node may also require attenuation, to adjust their optical power level.
In many of the optical network applications of Variable Optical Attenuators, it is necessary or desirable to monitor the optical power of the signal, after attenuation. (In some cases it may also be desirable to monitor the optical power of the signal prior to attenuation, although this is less common.) In many cases, it is desirable to be able to set the optical power at the VOA output to a specific power level. A closed-loop feedback system may be used to maintain the optical power of the signal at a specified power level, at the output of the VOA, even as the optical power of the signal at the VOA input changes. For this reason, it is common practice to use an optical tap and an optical power detector, at the output of a VOA. The optical tap splits off a small portion of the optical signal at the VOA's output. Depending on the optical power levels involved, and the amount of accuracy required, the percentage of the output signal's optical power that is split off might be 1%, 2%, 5%, or even 10%. The split-off optical signal is then directed to an optical detector device, which converts the optical power to an electrical signal, from which the optical power of the signal can be determined. This information can be used by the network operator, or an intelligent network controller, to set the attenuation level of the VOA, to maintain the optical power at the VOA output, to a specified level. The remainder of the optical signal at the VOA's output (the portion that was not split off and directed to the detector circuit) is than passed on to the rest of the network. The portion of the optical power that was split off by the optical splitter or tap represents a source of insertion loss to the desired/intended optical signal.
Similarly, optical switches are often used in optical networks, and in some cases it is desirable to monitor the optical power of the switched optical signal. For example, in order to monitor the optical power of multiple fibers, an N×1 selector switch may be used to connect one of the N fibers to an optical tap and photo-detector. 2×1 optical switches are frequently used to select between a primary fiber and a secondary or backup fiber, for improved reliability. In this application, it would also be desirable to be able to monitor the optical power of the selected fiber.
Optical splitters or couplers are widely available components that can be used to provide the optical tap function, with a variety of tap ratios (the percentage of the incoming optical power that is split off to one output of the splitter, with the remainder of the optical power being passed to the other output). Optical power detectors, such as photodiodes, are also widely available, with a variety of sensitivities. Optical tap-detectors are components that combine the functions of an optical tap, or splitter, with the optical detector.
FIG. 1 shows a prior art optical power equalizer, as used in a wavelength-division-multiplex (WDM) optical network. As multiple wavelengths propagate through fiber, optical amplifier(s), and other optical components, their individual power levels can vary significantly, as a result of wavelength-dependent attenuation and amplification. In order to transmit the optical signals over longer distances, while maintaining a high signal-to-noise ratio before entering the receiver, the optical power of the individual wavelengths needs to be equalized, at some point along the optical transmission path. In FIG. 1, the exemplary four wavelengths λ1 through λ4, having unequal individual power levels, enter a demultiplexer (marked as DEMUX) and then branch out as four separate wavelengths. Using the optical path of λ1 as an example, a small percentage of light from the main path 110 is tapped by an optical tap coupler 105, made by fusing two fibers together (as illustrated subsequently in FIG. 4), or via the use of a thin-film optical filter, or other technology. The tapped signal is than sent to a photo-detector 106 to monitor the power level of the line. A Variable Optical Attenuators (VOA) 104 is adjusted by a dynamic feedback circuit until the desired optical power level is achieved. The same approach is applied to the other three wavelengths, or channels. Therefore, when the four wavelengths are combined by the wavelength multiplexer (marked as MUX), their individual optical power levels are equal.
FIG. 2 shows a similar prior art application of a VOA 207, tap coupler 208, and photo-detector 207, to monitor and control the optical power at the output of an Erbium-doped fiber amplifier, or EDFA. Because the inherent gain or optical amplification provided by an EDFA may be variable and difficult to control, and the optical power level of the input signal 201 may also vary significantly, the combination of the VOA, tap coupler, and photo-detector allows for a more precise setting of the optical power of the output signal 220.
As has been occurring with cell phones, and other forms of modern electronic devices, more and more optical components are being squeezed into individual optical modules, or more highly-integrated optical components, in order to save space, reduce cost, and also to upgrade the performance of optical networks and network control centers. Fiber splicing between separate fiber optic components is cumbersome, and also occupies space. Consequently, optical networks could be improved by integrating multiple optical components into a single package. Since Variable Optical Attenuators (VOAs) are frequently used with optical tap detectors, such networks could be improved by integrating the functions of optical power attenuation and optical power monitoring into a single, integrated optical component. Similarly, it can be useful to be able to integrate the functions of optical switching and optical power monitoring, into a single, integrated optical component.